1. Field of the Invention
The invention relates to an amplifier with substantially fixed input impedance and related method, and more particularly, to an amplifier and related method for utilizing a plurality of resistive negative feedback circuits to process a plurality of feedback signals so as to keep the input impedance of the amplifier substantially fixed in various gain modes.
2. Description of the Prior Art
Low-noise amplifiers (LNA) are indispensable elements in a receiver of a wireless communications system and used for providing signals received by an antenna with gains and sensitivity. Since the low-noise amplifier is installed in a front end of the receiver to process generally very weak signals, the performances of the low-noise amplifier, such as the noise figure, the RF gain, and the non-linearity, are highly related to the gross performance of receiver.
Please refer to FIG. 1, which is a functional block diagram of a receiver 10 in a wireless communications system. The receiver 10 in the present embodiment is mainly applied to the wireless communications system operating at 0.9 GHz to 10 GHz. Nowadays, most commercial wireless communications systems, such as GSM, Blue-tooth, and WLAN, operate around the frequency region (0.9 GHz to 10 GHz). The receiver 10 includes an antenna 12, a filter 14, a low-noise amplifier 16, a mixer 18, a local oscillator generator 20, and a signal processing module 22. The antenna 12 is used to receive an RF signal RF. After the RF signal RF is obtained from the antenna 12, the filter 14 will operate a frequency selection process for the RF signal RF to generate an input signal Si. The low-noise amplifier 16 then amplifies the input signal Si by a predetermined gain ratio. Since the received RF signal RF and the filtered input signal Si are very weak, the low-noise amplifier 16 installed after the filter 14 should bring very low noise. Afterwards, the input signal SI is outputted from the low-noise amplifier 16 and down-sampled to a specific frequency by the mixer 18 and the local oscillator generator 20. The signal processing module 22 will proceed with advanced operations such as demodulation.
When being implemented, under various conditions, the receiver 10 of the wireless communications system cannot receive the RF signal RF with fixed magnitude. Taking the signal transmission in a cellular phone as an instance, when the receiver 10 approaches the signal emitting end, such as a base station, the magnitude of the RF signal RF is higher than that when the receiver 10 is far away from the signal emitting end. Since the exceeding RF signal RF may saturate the system and disable the amplifier to linearly amplify signals, the low-noise amplifier 16 is generally designed as a variable gain amplifier operating in a plurality of gain modes. In the following statement, the variable gain amplifier can operate in two gain modes: a high-gain mode and a low-gain mode. As shown in FIG. 1, when the input signal SI is small, the low-noise amplifier 16 operates in the high-gain mode, and the input signal SI is amplified by a higher gain ratio and then outputted. On the other hand, when the input signal SI is large, the low-gain mode is applied to process the input signal SI for avoiding the saturation of the low-noise amplifier 16. Please refer to FIG. 2, which is a functional block diagram of an embodiment of the prior-art low-noise amplifier 16 shown in FIG. 1. The low-noise amplifier 16 is a variable gain amplifier, which can operate respectively in a high-gain mode and a low-gain mode. The low-noise amplifier 16 includes an input port 32, a gain circuit 34, and an output port 36. The input port 32 is used to receive the input signal Si, and the gain circuit 34 includes transistors Q1–Q5 and adjustable three biases B1–B3. The gain circuit 34 can be used to amplify the input signal SI by two corresponding (high/low) gain ratios respectively in the two (high/low) gain modes. The output port 36 is used to output the input signal SI amplified by the gain circuit 34.
Please continue to refer to FIG. 2. When the low-noise amplifier 16 operates in the high-gain mode, the bias B2 is higher than the bias B3, the transistors Q1, Q2, Q4, Q5 turn on, and the transistor Q3 turns off. The input signal SI is amplified through the transistors Q1, Q2, Q4, Q5 in the gain circuit 34 and outputted by the output port 36. When the low-noise amplifier 16 operates in the low-gain mode, the bias B3 is higher than the bias B2, the transistors Q1, Q3, Q4, Q5 turn on, and the transistor Q2 turns off. The input signal SI is amplified through the transistors Q4, Q5 and outputted by the output port 36. By initially making the sizes of the transistors Q1, Q2, Q3 larger than those of the transistors Q4, Q5, only a little part of the input signal SI passes the transistor Q4, Q5 to the output port 36, while most of the input signal SI passes the transistors Q1, Q3 to a voltage source VCC. Therefore, the switch between the high-gain mode and the low-gain mode relies on the comparison between the bias B2 and the bias B3 with constant bias B1. When being implemented, the bias B2 remains at a predetermined voltage value, while the bias B3 is switched between two values (higher/lower than the bias B2).
In addition, an amplifier generally includes an input impedance and an output impedance. In a system, once the amplifier is electrically connected to other circuitries, a loading effect may occur to affect the performances of the whole system due to the (input/output) impedance mismatch between the amplifier and other circuitries. Please refer to both FIG. 1 and FIG. 2. The low-noise amplifier 16 includes an input impedance Zin1, an inductive negative feedback circuit 38, and an inductive loading Lc. For avoiding the impedance mismatch between the filter 14 and the low-noise amplifier 16 to affect the response of the filter 14 and the performance of the low-noise amplifier 16, in the prior-art embodiment, the emitters of the transistor Q1, Q4 are electrically connected to the inductive negative feedback circuit 38 in order to adjust the input impedance Zin1. Therefore, when the low-noise amplifier 16 is switched between the high-gain mode and the low-gain mode, the response of the filter 14 will not change due to the change of the input impedance Zin1 and thus the performance of the receiver 10 can be maintained.
However, since the circuit area of the inductive negative feedback circuit 38 is too large, concerning the cost, the resistive loading and the resistive negative feedback circuit are more acceptable for the industry. Please refer to FIG. 3, which is a functional block diagram of another embodiment of the prior-art low-noise amplifier 16 shown in FIG. 1. The low-noise amplifier 16 is still a variable gain amplifier operating in the high-gain mode and the low-gain mode. Similar to the embodiment shown in FIG. 2, the low-noise amplifier 16 consists of the input port 32, the gain circuit 34, the output port 36, and an input impedance Zin1. The gain circuit 34 includes the transistors Q1–Q5 and adjustable three biases B1-B3″ for amplifying the input signal SI by corresponding two (high/low) gain ratios respectively in the two (high/low) gain modes. The main difference between the present embodiment and the previous one is that in the present embodiment, a resistive loading RL and a resistive negative feedback circuit 40 substitute the inductive loading Lc and the inductive negative feedback circuit 38 shown in FIG. 2 to achieve a negative feedback function. For clarifying the characteristics of the resistive negative feedback, we take another amplifier as an instance. Please refer to FIG. 4, which is an (simple) amplifier 50 combined with a resistor Rf used for resistive negative feedback circuit. The amplifier 50 consists of a transistor Q6, an input port 52, an output port 56, an effective resistor R, and a resistor Rf for negative feedback. Without the resistor Rf (for negative feedback) involved, the amplifier has the voltage gain,Av1=gm.·Rwherein gm is a characteristic parameter of the transistor Q6. As shown in FIG. 4, the real line shows a frequency response of the amplifier 50 without the negative feedback (the horizontal axis represents the frequency f, and the vertical axis represents the gain Av). With the resistor Rf involved, the gain diminishes toAv2≈gm·R·Rf/(Rf+R),and the dotted line shown in FIG. 4 represents the frequency response of the amplifier 50 with the negative feedback. As shown in FIG. 4, the diminished gain (reduced byRf/(Rf+R)))can instead bring a better frequency response for the gain ratio of the frequency response remains flat over a wider frequency range. In the meantime, the distortion is also reduced along with the reduction of the gain ratio. In addition, the input impedance Zin2 of the amplifier 50 is changed toZin2≈(Rf+R)/(gm·R)by the effect of the resistor Rf; that is, the resistive negative feedback circuit 40 can be used to adjust the input impedance Zin2 of the amplifier.
Please refer back to FIG. 3. The resistive negative feedback circuit 40 is a resistor R electrically connected to a capacitor C. The embodiment shown in FIG. 3 operates similarly to the embodiment shown in FIG. 2. When the low-noise amplifier 16 operates in the high-gain mode, partial input signal SI processed and outputted to the output port 36 will be fed back from the output port 36 to the input port 32 via the resistive negative feedback circuit 40, called a feedback signal. However, when the low-noise amplifier 16 is switched to the low-gain mode, a little input signal SI will pass the transistors Q4′, Q5′ to the output port 36, while most of the input signal SI pass the transistors Q1′, Q3′ to a voltage source VCC′; that is, only a little feedback signal passes the resistive negative feedback circuit 40 back to the input port 32. Therefore, in various gain modes, the resistive negative feedback circuit 40 cannot be used to adjust the input impedance so that the input impedance Zin1′ ill alter in different gain modes. As we know, the filtering response of the filter 14 will be distorted by the impedance mismatch between the filter 14 and the low-noise amplifier 16, and the performances of the low-noise amplifier 16 will be affected.